Global Systems for Mobile (GSM):

GSM is a digital mobile telephony system that is widely used in Europe and other parts of the world. It digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either 900 MHz or or 1800 MHz frequency band.

How does it work?

GSM works when a mobile phone attaches to a mobile network it provides a globally unique identifier called the IMSI that it stored on the SIM card. The network serving your phone uses the first digits of the IMSI to find your home network’s identifier and it then queries a database called the Home Subscriber Subsystem (HSS) to fetch your records along with a magic pair of numbers used to authenticate you. The visited network then sends your phone one of these numbers and your SIM card calculates an answer that should be the same as the other number. If they match, then you can get service and your HHS record is updated to point to your current visited network. From here you can now place calls and the visited network knows it will be paid to carry them after it bills your home network.